One-part structural epoxy resin adhesives containing elastomeric tougheners capped with phenols and hydroxy-terminated acrylates or hydroxy-terminated methacrylates

ABSTRACT

Structural adhesives are prepared from an elastomeric toughener that contains urethane and/or urea groups, and have some terminal isocyanate groups that are capped with a phenol and other terminal isocyanate groups that are capped with a hydroxy-functional acrylate or a hydroxy-functional methacrylate. In certain embodiments, the presence of both types of capping on the toughener leads to higher impact peel strengths and a greater level of cohesive failure, than when the toughener is capped with a phenol an hydroxy-functional acrylate or hydroxy-functional methacrylate alone.

This application claims priority from U.S. Provisional Application No.61/087,808, filed 11 Aug. 2008.

This invention relates to an epoxy-based structural adhesive containingan elastomeric toughener having terminal isocyanate groups, some ofwhich are blocked with phenols and others of which are blocked with ahydroxy-terminated acrylate or methacrylate compound

Epoxy resin based adhesives are used in many applications. In theautomotive industry, epoxy resin adhesives are used in many bondingapplications, including metal-metal bonding in frame and otherstructures in automobiles. Some of these adhesives must strongly resistfailure during vehicle collision situations. Adhesives of this type aresometimes referred to as “crash durable adhesives”, or “CDAs”.

In order to obtain the good balance of properties that are needed tomeet stringent automotive performance requirements, epoxy adhesives areoften formulated with various rubbers and/or “tougheners”. Thesetougheners have blocked functional groups which, under the conditions ofthe curing reaction, can become de-blocked and react with an epoxyresin. Tougheners of this type are described, for example, in U.S. Pat.Nos. 5,202,390, 5,278,257, WO 2005/118734, U.S. Published PatentApplication No. 2005/0070634, U.S. Published Patent Application No.2005/0209401, U.S. Published Patent Application 2006/0276601 and EP-A-0308 664.

A commonly used toughener is a polyurethane and/or polyurea which iscapped with phenolic groups. These tougheners have been used with somesuccess in CDA applications. However, there is in some cases a desire tofurther increase the strength of these adhesives and to develop anadhesive which, when cured, fails more often in a cohesive failure moderather than an adhesive failure mode.

Another type of toughener is an acrylate-terminated polyurethane and/orpolyurea. These have been described in WO 03/078163 for use inone-component epoxy adhesives, and in U.S. Pat. Nos. 5,232,996,6,660,805 and US Published Patent Application 2004/0229990 for use intwo-part adhesives. The acrylate-terminated tougheners generally performmore poorly than do the phenolic-terminated types in CDA applications.Their use tends to lead to poorer lap shear and impact peel strengths inthe cured adhesives. The adhesives containing the acrylate-terminatedtougheners also tend to fail in an adhesive mode rather than the moredesirable cohesive mode.

A third type of toughener is described, for example, in EP 1,431,325, EP1,498,441, EP 1,648,950 and EP 1,741,734. These tougheners areepoxide-terminated, rather than being terminated with phenols oracrylates. Similar to the acrylate-terminated types, adhesivescontaining these tougheners tend to have poorer adhesive strengths andto fail mainly in the less desired adhesive failure mode.

EP 1916269 describes reactive tougheners having both epoxide andphenolic terminal groups, in a molar ratio of up to 20:80. These areprepared by reacting an isocyanate-terminated prepolymer sequentiallywith a hydroxyl-containing epoxide compound and cashew nut oil ordiallylbisphenol-A.

This invention is an elastomeric reactive toughener containing urethaneand/or urea groups, and which has capped terminal isocyanate groups,wherein a portion of the isocyanate groups are capped with a phenol andanother portion of the isocyanate groups are capped with ahydroxy-functional acrylate or a hydroxy-functional methacrylate.

In another aspect, this invention is a structural adhesive, comprising:

-   A) at least one epoxy resin;-   B) a reactive toughener containing urethane and/or urea groups, and    which has capped terminal isocyanate groups, wherein a portion of    the isocyanate groups are capped with a phenol and another portion    of the isocyanate groups are capped with a hydroxy-functional    acrylate or a hydroxy-functional methacrylate; and-   C) one or more epoxy curing agents.

The invention is also a method comprising applying the foregoingstructural adhesive to the surfaces of two members, and curing thestructural adhesive to form an adhesive bond between the two members. Atleast one and preferably both of the members are metals.

Cured adhesives containing this toughener often have excellent adhesiveproperties, including good lap shear strength and impact peel strength.The cured adhesives often are more likely to fail in a cohesive moderather than are adhesives that contain only one type of the cappinggroup. These advantages are seen most clearly in preferred embodimentsin which from 1 to 20 percent of the terminal isocyanate groups arecapped with a hydroxy-functional acrylate or hydroxy-functionalmethacrylate, and the remainder are capped with the phenol. Inespecially preferred embodiments, from 1.5 to 5 percent of the terminalisocyanate groups are capped with a hydroxy-functional acrylate ormethacrylate, and the remainder are capped with the phenol.

The toughener of the invention is elastomeric, contains urethane and/orurea groups and has terminal isocyanate groups, some of which are cappedwith a phenol and some of which are capped with a hydroxy-functionalacrylate or a hydroxy-functional methacrylate as described herein. Forconvenience herein, isocyanate groups capped with hydroxy-functionalacrylate or methacrylate groups are collectively referred to by theshorthand “(meth)acrylate-capped” isocyanate groups. From 1 to 99% ofthe terminal isocyanate groups in the elastomeric toughener can be(meth)acrylate-capped, and, correspondingly, from 1 to 99% of theterminal isocyanate groups can be capped with the phenol.

In cases in which some of the terminal isocyanate groups are(meth)acrylate-capped, it is preferred that from 1 to 20% of theterminal isocyanate groups are (meth)acrylate-capped, with the restbeing capped with the phenol. In this range, adhesive strength of thecured adhesive (i.e., lap shear strength and impact peel strength) tendsto be significantly higher than when a greater proportion of theisocyanate groups are (meth)acrylate-capped. In addition, the failuremode in these cases tends to more highly favor the desired cohesivefailure mode.

In some embodiments, from 1.5 to 5% of the terminal isocyanate groupsare (meth)acrylate-capped, with the rest being capped with the phenol.In this range, it has been found, surprisingly, that adhesive strengthof the cured adhesive can meet or even exceed that of an adhesivecontaining a toughener which is entirely phenol-capped. In addition,cohesive failure rates tend to be highest in this range, and oftenexceed those obtained by using an entirely phenol-capped toughener.

It will of course be recognized that the aforementioned proportions ofphenol-capped isocyanate groups and (meth)acrylate-capped isocyanategroups refer to averages among the toughener molecules, and not toindividual molecules specifically. Individual molecules may contain onlyphenol-capped isocyanate groups or only (meth)acrylate-capped isocyanategroups, or some of each of these types in any proportion. The phenol-and (meth)acrylate-capping on individual molecules will usually bedistributed statistically, and may depend on the particular methods usedto prepare the toughener.

The toughener suitably contains, on average, from about 1.5, preferablyfrom about 2.0, to about 8, preferably to about 6, more preferably toabout 4, capped isocyanate groups per molecule.

The toughener contains at least one internal segment that provideselastomeric character. It may contain two or more such segments. Thissegment may be a polyether segment or a segment of a butadienehomopolymer or copolymer. Segments of both types may be present in thetoughener. Each polyether segment or segment of a butadiene homopolymeror copolymer preferably has a weight of from 800 to 5000 daltons,preferably from 1500 to 4000 daltons, measured by GPC.

The toughener suitably has a number average molecular weight from atleast 3000, preferably at least 5000, to about 30,000, preferably toabout 20,000 and more preferably to about 15,000, measured by GPC. Thepolydispersity (ratio of weight average molecular weight to numberaverage molecular weight) is suitably from about 1 to about 4,preferably from about 1.5 to 2.5.

The toughener may also contain residues of a branching agent, a chainextender, or both.

The elastomeric toughener can be represented by the idealized structure(I)

wherein p represents the average number of capped isocyanate groups permolecule. p is suitably at least 1.5, preferably at least 2, to 8,preferably to 6, more preferably to 4. Each A in structure I representsthe residue, after removal of a hydrogen atom, from (1) an amino groupof a primary aliphatic, cycloaliphatic, heterocyclic or araliphaticamine; an amino group of a secondary aliphatic, cycloaliphatic,heterocyclic or araliphatic amine or a hydroxyl group of ahydroxy-functional acrylate or methacrylate, or (2) a phenol.

On average, the elastomeric toughener in structure I will include someof both of type (1) and type (2) A groups. Individual molecules of theelastomeric toughener may have only type (1) A groups, only type (2) Agroups, or both type (1) and type (2) A groups. The type (1) and (2) Agroups can be represented by structures II and III, respectively:

In structure II, R¹ is hydrogen or methyl. In structure III, m is anumber from 0 to 5. Each R³ represents a substituent group, bonded to acarbon atom on the aromatic ring. Each R³ may be a hydroxyl group; analkyl group, which may be linear, branched or cycloalkyl; an alkenylgroup such as allyl; an aromatic group such as phenyl, alkyl-substitutedphenyl, alkenyl-substituted phenyl and the like; an aryl-substitutedalkyl group; a phenol-substituted alkyl group, wherein the phenolsubstituent group may itself be unsubstituted or substituted; and thelike.

In structure I, Y is the residue of an isocyanate-terminated prepolymerafter removal of the terminal isocyanate groups. Y contains at least oneelastomeric segment. Each elastomeric segment preferably has arelatively high weight, preferably a weight of at least 800 daltons. Theweight of the elastomeric segment may be as high as 5000 daltons, and ispreferably from 1500 to 4000 daltons, in each case. This elastomericsegment is preferably linear or at most slightly branched. Theelastomeric segment(s) each may be a polyether segment or a segment of abutadiene homopolymer or copolymer, as described before. The Y group maycontain one or more segments of each type. The Y group may containurethane and/or urea groups, and may in addition contain residues (afterremoval of hydroxyl or amino groups, as the case may be) of one or morecrosslinkers or chain extenders. Crosslinkers, for purposes of thisinvention, are polyol or polyamine compounds having a molecular weightof up to 750, preferably from 50 to 500, and at least three hydroxyl,primary amino and/or secondary amino groups per molecule. Crosslinkersprovide branching to the Y group, and are useful to increase thefunctionality (i.e., number of capped isocyanate groups per molecule) ofthe toughener. Chain extenders, for purposes of this invention, arepolyol or polyamine compounds having a molecular weight of up to 750,preferably from 50 to 500, and two hydroxyl, primary amino and/orsecondary amino groups per molecule. Chain extenders help to increasethe molecular weight of the toughener without increasing functionality.

The reactive toughener can be prepared by forming anisocyanate-terminated prepolymer, and then capping some of the terminalisocyanate groups with a phenol compound and the remainder of theterminal isocyanate groups with a hydroxy-functional acrylate and/or ahydroxy-functional methacrylate compound. The isocyanate-terminatedprepolymer can be prepared by reaction of one or more polyol orpolyamine compounds with a stoichiometric excess of a polyisocyanatecompound, preferably a diisocyanate compound. At least one of the polyolor polyamine compounds imparts elastomeric properties to the toughener,and preferably includes a relatively high weight elastomeric segment,especially a polyether segment or a segment of a butadiene homopolymeror copolymer, as described before.

The polyisocyanate may be an aromatic polyisocyanate, but it ispreferably an aliphatic polyisocyanate such as isophorone diisocyanate,hexamethylene diisocyanate, hydrogenated toluene diisocyanate,hydrogenated methylene diphenylisocyanate (H₁₂MDI), and the like.

In the simplest case, only one polyol or polyamine is used to make theprepolymer. In such a case, the polyol or polyamine preferably containsat least one relatively high weight elastomeric segment as describedbefore, to impart elastomeric properties to the prepolymer. However, itis also possible to use a mixture of polyols or polyamines to make theprepolymer. It is preferred that at least 50%, more preferably at least80%, and even more preferably at least 90%, by weight of the polyol orpolyamine materials used to make the prepolymer include a relativelyhigh weight elastomeric segment as described before.

Other polyols and polyamines that can be used in combination with theelastomeric polyol or polyamine(s) include crosslinkers and chainextenders having a molecular weight of up to about 750. More preferredare aliphatic polyols and polyamines having an equivalent weight of upto 150 and from 2 to 4, especially from 2 to 3, hydroxyl and/or primaryor secondary amino groups. Examples of these materials include polyolssuch as trimethylolpropane, glycerine, trimethylolethane, ethyleneglycol, diethylene glycol, propylene glycol, dipropylene glycol,sucrose, sorbitol, pentaerythritol, ethylene diamine, triethanolamine,monoethanolamine, diethanolamine, piperazine, aminoethylpiperazine,compounds having two or more phenolic hydroxyl groups such resorcinol,catechol, hydroquinone, bisphenol, bisphenol A, bisphenol AP(1,1-bis(4-hydroxylphenyl)-1-phenyl ethane), bisphenol F, bisphenol K,bisphenol M, tetramethylbiphenol and o,o′-diallyl-bisphenol A, and thelike.

When a mixture of polyols/or polyamines is used to make the prepolymer,the polyisocyanate compound can be reacted all at once with the mixtureto produce the prepolymer in a single step. Alternatively, thepolyisocyanate may be reacted with each polyol or polyamine compoundsequentially, or with various subsets thereof. The latter approach isoften useful to produce a prepolymer having a more defined molecularstructure.

Thus, for example, a prepolymer can be formed from one or more higherequivalent weight polyols or polyamines having one or more elastomericsegments, as described before, and one or more chain extenders orcrosslinkers. In such a case, the prepolymer can be made by reacting allof the polyols and/or polyamines, including chain extenders and/orcrosslinkers, at once with the polyisocyanate. Alternatively, acrosslinker or chain extender can be reacted first with thepolyisocyanate, followed by reaction with the higher equivalent weightpolyol and/or polyamine, or vice versa. In another approach, the higherequivalent weight polyol and/or polyamine, or mixture thereof with acrosslinker, is first reacted with the polyisocyanate, and the resultingproduct is then reacted with a chain extender or additional crosslinkerto advance the molecular weight.

The proportions of starting materials are suitably selected so that theprepolymer has an isocyanate content of from 0.5 to 6% by weight, morepreferably from 1 to 5% by weight and even more preferably from 1.5 to4% by weight. In terms of isocyanate equivalent weight, a preferredrange is from 700 to 8400, a more preferred range is from 840 to 4200,and an even more preferred range is from 1050 to 2800.

The elastomeric toughener is then prepared from the prepolymer byreacting the isocyanate prepolymer with a capping agent which includes(1) a phenol compound and (2) a hydroxy-functional acrylate or ahydroxy-functional methacrylate compound. These materials can be reactedwith the prepolymer either sequentially (in either order) orsimultaneously.

If the two types of capping agents are reacted sequentially with theprepolymer, it is preferred to react the prepolymer first with the typeof capping agent that will be used in the smaller molar proportion,followed by capping with the type of other capping agent. In thesecases, individual molecules may contain only one type of capping group,and other individual molecules may contain both types of capping groups,but the toughener as a whole will contain both types of capping groups.It is also possible to cap part of the toughener with one of the cappingagents and separately cap another part of the prepolymer with the othercapping agent. The two capped prepolymers are then combined to form thereactive toughener of the invention. In this case, some individualmolecules in the elastomeric toughener will have only one type ofcapping group, and other individual molecules will have the other typeof capping group.

The proportions of starting materials are selected so that at least onemole of capping agent is provided per equivalent of isocyanate group onthe prepolymer. Such a ratio of starting materials allows for thecapping reaction to proceed until the isocyanate groups are essentiallyall consumed, without significantly advancing the prepolymer.

The phenolic compound contains at least one phenolic hydroxyl group,i.e., a hydroxyl group bonded directly to a carbon atom of an aromaticring. The phenolic compound may have two or more phenolic hydroxylgroups, but preferably contains only one phenolic hydroxyl group. Thephenolic compound may contain other substituent groups, but thesepreferably are not reactive with an isocyanate group under theconditions of the capping reaction. Alkenyl groups, especially allylgroups, are of particular interest. Other suitable substituent groupsinclude alkyl groups, which may be linear, branched or cycloalkyl;aromatic groups such as phenyl, alkyl-substituted phenyl,alkenyl-substituted phenyl and the like; aryl-substituted alkyl groups;and phenol-substituted alkyl groups, wherein the phenol substituentgroup may itself be unsubstituted or substituted. Examples of suitablephenolic compounds include phenol, cresol, allylphenol (especiallyo-allylphenol), resorcinol, catechol, hydroquinone, bisphenol, bisphenolA, bisphenol AP (1,1-bis(4-hydroxylphenyl)-1-phenyl ethane), bisphenolF, bisphenol K, bisphenol M, tetramethylbiphenol ando,o′-diallyl-bisphenol A.

Suitable hydroxy-functional acrylate or methacrylate compounds includeacrylate or methacrylate compounds having one or more, especially one,hydroxyl group. Hydroxyl-functional acrylate and methacrylate compoundsare preferred. Among the suitable capping agents of this type are2-hydroxyethylacrylate, 2-hydroxypropylacrylate, 4-hydroxybutylacrylate,2-hydroxybutylacrylate, 2-aminopropylacrylate,2-hydroxyethylmethacrylate, 2-hydroxypropylmethacrylate,4-hydroxybutylmethacrylate, 2-hydroxybutylmethacrylate, and the like.

The prepolymer-forming and capping reactions are generally performed bymixing the starting materials in the presence of a catalyst for thereaction of isocyanate groups with hydroxyl and/or amino groups, as thecase may be. It is not usually necessary to catalyze a reaction betweenthe isocyanate groups and an amine. The reaction mixture will typicallybe heated to an elevated temperature, and the reaction continued untilthe isocyanate content is reduced to the desired level (approximately 0%for the capping reaction, somewhat higher levels for the prepolymer asdescribed above).

The toughener should constitute at least 5 weight percent of theadhesive composition. Better results are typically seen when the amountof toughener is at least 8 weight percent or at least 10 weight percent.The toughener may constitute up to 45 weight percent thereof, preferablyup to 30 weight percent and more preferably up to 25 weight percent. Theamount of toughener that is needed to provide good properties,particularly good low temperature properties, in any particular adhesivecomposition may depend somewhat on the other components of thecomposition, and may depend somewhat on the molecular weight of thetoughener.

The structural adhesive contains at least one epoxy resin. It ispreferred that at least a portion of the epoxy resin is notrubber-modified. A non-rubber-modified epoxy resin may be added to thestructural adhesive as a separate component, i.e., not as a component ofa rubber-modified epoxy resin or a dispersion of a core-shell rubber, asdescribed below. In some embodiments of the invention, a core-shellrubber product is used, which may be dispersed in some quantity of epoxyresin. Some amount of non-rubber-modified epoxy resin may be broughtinto the structural adhesive in that manner. In other embodiments, arubber-modified epoxy resin product used as a component of thestructural adhesive may also contain a certain amount of epoxy resinwhich is not reacted with the rubber (and thus is not rubber-modified).Some non-rubber-modified epoxy resin may be brought into the adhesive inthat manner as well.

A wide range of epoxy resins can be used as a non-rubber-modified epoxyresin, including those described at column 2 line 66 to column 4 line 24of U.S. Pat. No. 4,734,332, incorporated herein by reference.

Suitable epoxy resins include the diglycidyl ethers of polyhydric phenolcompounds such as resorcinol, catechol, hydroquinone, bisphenol,bisphenol A, bisphenol AP (1,1-bis(4-hydroxylphenyl)-1-phenyl ethane),bisphenol F, bisphenol K and tetramethylbiphenol; diglycidyl ethers ofaliphatic glycols and polyether glycols such as the diglycidyl ethers ofC₂₋₂₄ alkylene glycols and poly(ethylene oxide) or poly(propylene oxide)glycols; polyglycidyl ethers of phenol-formaldehyde novolac resins,alkyl substituted phenol-formaldehyde resins (epoxy novolac resins),phenol-hydroxybenzaldehyde resins, cresol-hydroxybenzaldehyde resins,dicyclopentadiene-phenol resins and dicyclopentadiene-substituted phenolresins; and any combination of any two or more thereof.

Suitable epoxy resins include diglycidyl ethers of bisphenol A resinssuch as are sold by Dow Chemical under the designations D.E.R.® 330,D.E.R.® 331, D.E.R.® 332, D.E.R.® 383, D.E.R. 661 and D.E.R.® 662resins.

Commercially available diglycidyl ethers of polyglycols that are usefulinclude those sold as D.E.R.® 732 and D.E.R.® 736 by Dow Chemical.

Epoxy novolac resins can be used. Such resins are available commerciallyas D.E.N.® 354, D.E.N.® 431, D.E.N.® 438 and D.E.N.® 439 from DowChemical.

Other suitable additional epoxy resins are cycloaliphatic epoxides. Acycloaliphatic epoxide includes a saturated carbon ring having an epoxyoxygen bonded to two vicinal atoms in the carbon ring, as illustrated bythe following structure IV:

wherein R is an aliphatic, cycloaliphatic and/or aromatic group and n isa number from 1 to 10, preferably from 2 to 4. When n is 1, thecycloaliphatic epoxide is a monoepoxide. Di- or polyepoxides are formedwhen n is 2 or more. Mixtures of mono-, di- and/or polyepoxides can beused. Cycloaliphatic epoxy resins as described in U.S. Pat. No.3,686,359, incorporated herein by reference, may be used in the presentinvention. Cycloaliphatic epoxy resins of particular interest are(3,4-epoxycyclohexyl-methyl)-3,4-epoxy-cyclohexane carboxylate,bis-(3,4-epoxycyclohexyl) adipate, vinylcyclohexene monoxide andmixtures thereof.

Other suitable epoxy resins include oxazolidone-containing compounds asdescribed in U.S. Pat. No. 5,112,932. In addition, an advancedepoxy-isocyanate copolymer such as those sold commercially as D.E.R. 592and D.E.R. 6508 (Dow Chemical) can be used.

The non-rubber-modified epoxy resin preferably is a bisphenol-type epoxyresin or mixture thereof with up to 10 percent by weight of another typeof epoxy resin. The most preferred epoxy resins are bisphenol-A basedepoxy resins and bisphenol-F based epoxy resins. These can have averageepoxy equivalent weights of from about 170 to 600 or more, preferablyfrom 225 to 400.

An especially preferred non-rubber-modified epoxy resin is a mixture ofa diglycidyl ether of a polyhydric phenol, preferably bisphenol-A orbisphenol-F, having an epoxy equivalent weight of from 170 to 299,especially from 170 to 225, and a second diglycidyl ether of apolyhydric phenol, again preferably bisphenol-A or bisphenol-F, this onehaving an epoxy equivalent weight of at least 300, preferably from 310to 600. The proportions of the two resins are preferably such that themixture of the two resins has an average epoxy equivalent weight of from225 to 400. The mixture optionally may also contain up to 20%,preferably up to 10%, of one or more other non-rubber-modified epoxyresins.

A non-rubber-modified epoxy resin preferably will constitute at leastabout 25 weight percent of the structural adhesive, more preferably atleast about 30 weight percent, and still more preferably at least about35 part weight percent. The non-rubber-modified epoxy resin mayconstitutes up to about 60 weight percent of the structural adhesive,more preferably up to about 50 weight percent. These amounts includeamounts of non-rubber-modified epoxy resin (if any) that may be broughtinto the composition with any core-shell rubber and/or any liquidrubber-modified epoxy resin(s) as may be used.

The structural adhesive also contains a curing agent. For the preferredone-component adhesive products, the curing agent preferably is selectedtogether with any catalysts such that the adhesive cures rapidly whenheated to a temperature of 80° C. or greater, preferably 140° C. orgreater, but cures very slowly if at all at room temperature (˜22° C.)and temperatures up to at least 50° C. Suitable such curing agentsinclude materials such as boron trichloride/amine and borontrifluoride/amine complexes, dicyandiamide, melamine, diallylmelamine,guanamines such as acetoguanamine and benzoguanamine, aminotriazolessuch as 3-amino-1,2,4-triazole, hydrazides such as adipic dihydrazide,stearic dihydrazide, isophthalic dihydrazide, semicarbazide,cyanoacetamide, and aromatic polyamines such asdiaminodiphenylsulphones. The use of dicyandiamide, isophthalic aciddihydrazide, adipic acid dihydrazide and/or 4,4′-diaminodiphenylsulphoneis particularly preferred.

The curing agent is used in an amount sufficient to cure thecomposition. Typically, enough of the curing agent is provided toconsume at least 80% of the epoxide groups present in the composition. Alarge excess over that amount needed to consume the epoxide groups isgenerally not needed. Preferably, the curing agent constitutes at leastabout 1.5 weight percent of the structural adhesive, more preferably atleast about 2.5 weight percent and even more preferably at least 3.0weight percent. The curing agent preferably constitutes up to about 15weight percent of the structural adhesive composition, more preferablyup to about 10 weight percent, and most preferably up to about 8 weightpercent.

The structural adhesive will in most cases contain a catalyst to promotethe cure of the adhesive, i.e. the reaction of epoxy groups withepoxide-reactive groups on the curing agent and other components of theadhesive. Among preferred epoxy catalysts are ureas such asp-chlorophenyl-N,N-dimethylurea (Monuron), 3-phenyl-1,1-dimethylurea(Phenuron), 3,4-dichlorophenyl-N,N-dimethylurea (Diuron),N-(3-chloro-4-methylphenyl)-N′,N′-dimethylurea (Chlortoluron),tert-acryl- or alkylene amines like benzyldimethylamine,2,4,6-tris(dimethylaminomethyl)phenol, piperidine or derivates thereof,imidazole derivates, in general C₁-C₁₂ alkylene imidazole orN-arylimidazols, such as 2-ethyl-2-methylimidazol, or N-butylimidazol,6-caprolactam, a preferred catalyst is2,4,6-tris(dimethylaminomethyl)phenol integrated into apoly(p-vinylphenol) matrix (as described in European patent EP 0 197892). The catalyst may be encapsulated or otherwise be a latent typewhich becomes active only upon exposure to elevated temperatures.

Preferably, the catalyst is present in an amount of at least about 0.1weight percent of the structural adhesive, and more preferably at leastabout 0.5 weight percent. Preferably, the catalyst constitutes up toabout 2 weight percent of the structural adhesive, more preferably up toabout 1.0 weight percent, and most preferably up to about 0.7 weightpercent.

The structural adhesive of the invention may include at least one liquidrubber-modified epoxy resin. A rubber-modified epoxy resin for purposesof this invention is a reaction product of an epoxy resin and at leastone liquid rubber that has epoxide-reactive groups, such as amino orpreferably carboxyl groups. The resulting adduct has reactive epoxidegroups which can be cured further when the structural adhesive is cured.It is preferred that at least a portion of the liquid rubber has a glasstransition temperature (T_(g)) of −40° C. or lower, especially −50° C.or lower. Preferably, each of the rubbers (when more than one is used)has a glass transition temperature of −25° C. or lower. The rubber T_(g)may be as low as −100° C. or even lower.

The liquid rubber is preferably a homopolymer or copolymer of aconjugated diene, especially a diene/nitrile copolymer. The conjugateddiene rubber is preferably butadiene or isoprene, with butadiene beingespecially preferred. The preferred nitrile monomer is acrylonitrile.Preferred copolymers are butadiene-acrylonitrile copolymers. The rubberspreferably contain, in the aggregate, no more than 30 weight percentpolymerized unsaturated nitrile monomer, and preferably no more thanabout 26 weight percent polymerized nitrile monomer.

The rubber preferably contains from about 1.5, more preferably fromabout 1.8, to about 2.5, more preferably to about 2.2, ofepoxide-reactive terminal groups per molecule, on average.Carboxyl-terminated rubbers are preferred. The molecular weight (M_(n))of the rubber is suitably from about 2000 to about 6000, more preferablyfrom about 3000 to about 5000.

Suitable carboxyl-functional butadiene and butadiene/acrylonitrilerubbers are commercially available from Noveon under the tradenamesHycar® 2000X162 carboxyl-terminated butadiene homopolymer, Hycar®1300X31, Hycar® 1300X8, Hycar® 1300X13, Hycar® 1300X9 and Hycar® 1300X18carboxyl-terminated butadiene/acrylonitrile copolymers. A suitableamine-terminated butadiene/acrylonitrile copolymer is sold under thetradename Hycar® 1300X21.

The rubber is formed into an epoxy-terminated adduct by reaction with anexcess of an epoxy resin. Enough of the epoxy resin is provided to reactwith substantially all of the epoxide-reactive groups on the rubber andto provide free epoxide groups on the resulting adduct withoutsignificantly advancing the adduct to form high molecular weightspecies. A ratio of at least two equivalents of epoxy resin perequivalent of epoxy-reactive groups on the rubber is preferred. Morepreferably, enough of the epoxy resin is used that the resulting productis a mixture of the adduct and some free epoxy resin. Typically, therubber and an excess of the polyepoxide are mixed together with apolymerization catalyst and heated to a temperature of about 100 toabout 250° C. in order to form the adduct. Suitable catalysts includethose described before. Preferred catalysts for forming therubber-modified epoxy resin include phenyl dimethyl urea and triphenylphosphine.

A wide variety of epoxy resins can be used to make the rubber-modifiedepoxy resin, including any of those described above. The epoxy resin maybe the same or different from that used to prepare the rubber-modifiedepoxy resin. Preferred polyepoxides are liquid or solid glycidyl ethersof a bisphenol such as bisphenol A or bisphenol F. Halogenated,particularly brominated, resins can be used to impart flame retardantproperties if desired. Liquid epoxy resins (such as DER™ 330 and DER™331 resins, which are diglycidyl ethers of bisphenol A available fromThe Dow Chemical Company) are especially preferred for ease of handling.

The rubber-modified epoxy resin(s), if present at all, may constituteabout 1 weight percent of the structural adhesive or more, preferably atleast about 2 weight percent. The rubber-modified epoxy resin mayconstitute up to about 25 weight percent of the structural adhesive,more preferably up to about 20 weight percent, and even more preferablyup to about 15 weight percent.

The structural adhesive of the invention may contain one or morecore-shell rubbers. The core-shell rubber is a particulate materialhaving a rubbery core. The rubbery core preferably has a T_(g) of lessthan −20° C., more preferably less than −50° C. and even more preferablyless than −70° C. The T_(g) of the rubbery core may be well below −100°C. The core-shell rubber also has at least one shell portion thatpreferably has a T_(g) of at least 50° C. By “core”, it is meant aninternal portion of the core-shell rubber. The core may form the centerof the core-shell particle, or an internal shell or domain of thecore-shell rubber. A shell is a portion of the core-shell rubber that isexterior to the rubbery core. The shell portion (or portions) typicallyforms the outermost portion of the core-shell rubber particle. The shellmaterial is preferably grafted onto the core or is crosslinked or both.The rubbery core may constitute from 50 to 95%, especially from 60 to90%, of the weight of the core-shell rubber particle.

The core of the core-shell rubber may be a polymer or copolymer of aconjugated diene such as butadiene, or a lower alkyl acrylate such asn-butyl-, ethyl-, isobutyl- or 2-ethylhexylacrylate. The core polymermay in addition contain up to 20% by weight of other copolymerizedmonounsaturated monomers such as styrene, vinyl acetate, vinyl chloride,methyl methacrylate, and the like. The core polymer is optionallycrosslinked. The core polymer optionally contains up to 5% of acopolymerized graft-linking monomer having two or more sites ofunsaturation of unequal reactivity, such as diallyl maleate, monoallylfumarate, allyl methacrylate, and the like, at least one of the reactivesites being non-conjugated.

The core polymer may also be a silicone rubber. These materials oftenhave glass transition temperatures below −100° C. Core-shell rubbershaving a silicone rubber core include those commercially available fromWacker Chemie, Munich, Germany, under the trade name Genioperl™.

The shell polymer, which is optionally chemically grafted or crosslinkedto the rubber core, is preferably polymerized from at least one loweralkyl methacrylate such as methyl-, ethyl- or t-butyl methacrylate.Homopolymers of such methacrylate monomers can be used. Further, up to40% by weight of the shell polymer can be formed from othermonovinylidene monomers such as styrene, vinyl acetate, vinyl chloride,methyl acrylate, ethyl acrylate, butyl acrylate, and the like. Themolecular weight of the grafted shell polymer is generally between20,000 and 500,000.

A preferred type of core-shell rubber has reactive groups in the shellpolymer which can react with an epoxy resin or an epoxy resin hardener.Glycidyl groups such as are provided by monomers such as glycidylmethacrylate are suitable.

A particularly preferred type of core-shell rubber is of the typedescribed in EP 1 632 533 A1. Core-shell rubber particles as describedin EP 1 632 533 A1 include a crosslinked rubber core, in most casesbeing a crosslinked copolymer of butadiene, and a shell which ispreferably a copolymer of styrene, methyl methacrylate, glycidylmethacrylate and optionally acrylonitrile. The core-shell rubber ispreferably dispersed in a polymer or an epoxy resin, also as describedin EP 1 632 533 A1.

Preferred core-shell rubbers include those sold by Kaneka Corporationunder the designation Kaneka Kane Ace, including Kaneka Kane Ace MX 156and Kaneka Kane Ace MX 120 core-shell rubber dispersions. The productscontain the core-shell rubber particles pre-dispersed in an epoxy resin,at a concentration of approximately 25%. The epoxy resin contained inthose products will form all or part of the non-rubber-modified epoxyresin component of the structural adhesive of the invention.

The core-shell rubber particles can constitute from 0 to 15 weightpercent of the structural adhesive.

The total rubber content of the structural adhesive of the invention canrange from as little as 0 weight percent to as high as 30 weightpercent. A preferred rubber content for a crash durable adhesive is from1 weight percent to as much as 20 weight percent, preferably from 2 to15 weight percent and more preferably from 4 to 15 weight percent.

Total rubber content is calculated for purposes of this invention bydetermining the weight of core-shell rubber (if any), plus the weightcontributed by the liquid rubber portion of any rubber-modified epoxyresin as may be used. No portion of the elastomeric toughener isconsidered in calculating total rubber content. In each case, the weightof unreacted (non-rubber-modified) epoxy resins and/or other carriers,diluents, dispersants or other ingredients that may be contained in thecore-shell rubber product or rubber-modified epoxy resin is notincluded. The weight of the shell portion of the core-shell rubber iscounted as part of the total rubber content for purposes of thisinvention.

The structural adhesive of the invention may contain various otheroptional components.

The presence of acrylate or methacrylate capping groups in the toughenergives rise to the possibility of at least partially curing thestructural adhesive through a free radical polymerization of thecarbon-carbon double bonds in those groups. Therefore, a free radicalinitiator may be included in the structural adhesive. The free radicalinitiator should be a latent type that is activated at an elevatedtemperature, which is preferably from about 50° C. to about 150° C.

If the free radical initiator is activated at a lower temperature thanthat at which the epoxy resin cures, it becomes possible to conduct apartial cure of the structural adhesive through selective polymerizationof the acrylate or methacrylate capping groups. This may be done, forinstance, to increase the viscosity of the adhesive after application orto partially gel it, to form a temporary bond which can hold thesubstrates together until the final cure can be performed. In such acase, the free radical initiator preferably is activated by heating itto a temperature of from 80° C. to 130° C., more preferably from 100° C.to 120° C.

Acrylate or methacrylate capping groups also can be polymerized byexposure to activating radiation such as ultraviolet radiation, with orwithout the presence of a free radical initiator.

The speed and selectivity of the cure can be enhanced and adjusted byincorporating a monomeric or oligomeric, addition polymerizable,ethylenically unsaturated material into the structural adhesive. Thismaterial should have a molecular weight of less than about 1500. Thismaterial may be, for example, an acrylate or methacrylate compound, anunsaturated polyester, a vinyl ester resin, or an epoxy adduct of anunsaturated polyester resin. A free radical initiator as described abovecan be included in the structural adhesive as well, in order to providea source of free radicals to polymerize this material. The inclusion ofan ethylenically unsaturated material of this type again provides thepossibility of effecting a partial cure of the structural adhesivethrough selective polymerization of the ethylenic unsaturation. Thisadditional ethylenically unsaturated material can copolymerize with thetoughener.

At least one filler, rheology modifier and/or pigment is preferablypresent in the structural adhesive. These can perform several functions,such as (1) modifying the rheology of the adhesive in a desirable way,(2) reducing overall cost per unit weight, (3) absorbing moisture oroils from the adhesive or from a substrate to which it is applied,and/or (4) promoting cohesive, rather than adhesive, failure. Examplesof these materials include calcium carbonate, calcium oxide, talc,carbon black, textile fibers, glass particles or fibers, aramid pulp,boron fibers, carbon fibers, mineral silicates, mica, powdered quartz,hydrated aluminum oxide, bentonite, wollastonite, kaolin, fumed silica,silica aerogel or metal powders such as aluminum powder or iron powder.Another filler of particular interest is a microballon having an averageparticle size of up to 200 microns and density of up to 0.2 g/cc. Theparticle size is preferably about 25 to 150 microns and the density ispreferably from about 0.05 to about 0.15 g/cc. Heat expandablemicroballoons which are suitable for reducing density include thosecommercially available from Dualite Corporation under the tradedesignation Dualite™ and those sold by Akzo Nobel under the tradedesignation Expancel™.

Fillers, pigment and rheology modifiers are preferably are used in anaggregate amount of about 2 parts per hundred parts of adhesivecomposition or greater, more preferably about 5 parts per hundred partsof adhesive composition or greater. They preferably are present in anamount of up to about 25 weight percent of the structural adhesive, morepreferably up to about 20 weight percent, and most preferably up toabout 15 weight percent.

The structural adhesive can further contain other additives such asdiluents, plasticizers, extenders, pigments and dyes, fire-retardingagents, thixotropic agents, expanding agents, flow control agents,adhesion promoters and antioxidants. Suitable expanding agents includeboth physical and chemical type agents. The adhesive may also contain athermoplastic powder such as polyvinylbutyral or a polyester polyol, asdescribed in WO 2005/118734.

Various preferred adhesives of the invention are as follows:

A. An adhesive that includes at least one diglycidyl ether of apolyhydric phenol; a liquid rubber-modified epoxy resin, a core-shellrubber or both a liquid rubber-modified epoxy resin and a core-shellrubber; and from about 8 to 30 weight percent of the toughener of theinvention.

B. An adhesive as in A, wherein from 1 to 20% of the terminal isocyanategroups of the toughener are (meth)acrylate- and/or epoxide-capped, andthe remainder of terminal isocyanate groups are capped with a phenol.

C. An adhesive as in A or B, wherein the diglycidyl ether of apolyhydric phenol is a diglycidyl ether of bisphenol A or bisphenol F,and has an equivalent weight of from 170 to 299.

D. An adhesive as in A or B, which contains a mixture of a diglycidylether of a polyhydric phenol, preferably bisphenol-A or bisphenol-F,having an epoxy equivalent weight of from 170 to 299, especially from170 to 225, and a second diglycidyl ether of a polyhydric phenol, againpreferably bisphenol-A or bisphenol-F, this one having an epoxyequivalent weight of at least 300, preferably from 310 to 600. Theproportions of the two resins are preferably such that the mixture ofthe two resins has an average epoxy equivalent weight of from 225 to400.

E. An adhesive as in A, B, C, or D, wherein the toughener contains oneor more polyether segments of from 800 to 5000 daltons each.

F. An adhesive as in A, B, C, D or E, which is a one-part adhesive thatcontains a curing agent and a catalyst, and which cures rapidly at atemperature of 140° C. or higher but slowly if at all at a temperatureof 80° C. or lower.

G. An adhesive as in A, B, C, D, E or F, wherein from 1.5 to 5% of theterminal isocyanate groups of the toughener are (meth)acrylate-capped,and the remainder of terminal isocyanate groups are capped with aphenol.

The adhesive composition can be applied by any convenient technique. Itcan be applied cold or be applied warm if desired. It can be applied byextruding it from a robot into bead form on the substrate, it can beapplied using mechanical application methods such as a caulking gun, orany other manual application means, and it can also be applied using jetspraying methods such as a steaming method or a swirl technique. Theswirl technique is applied using an apparatus well known to one skilledin the art such as pumps, control systems, dosing gun assemblies, remotedosing devices and application guns. Preferably, the adhesive is appliedto the substrate using a jet spraying or streaming process. Generally,the adhesive is applied to one or both substrates. The substrates arecontacted such that the adhesive is located between the substrates to bebonded together.

After application, the structural adhesive is cured by heating to atemperature at which the curing agent initiates cure of the epoxy resincomposition. Generally, this temperature is about 80° C. or above,preferably about 140° C. or above. Preferably, the temperature is about220° C. or less, and more preferably about 180° C. or less.

It is possible to cure the structural adhesive in stages. The firststage is conducted at a somewhat lower temperature, such as from 50° C.to 100° C., preferably from 50 to 80° C. Ethylenic unsaturation(including acrylate or methacrylate capping groups on the reactivetoughener) can polymerize at these temperatures if a source of freeradicals is present, to partially cure the structural adhesive. Thepartially cured adhesive then can be subjected to a higher temperatureat which the epoxy resin can react with the epoxy curing agent tocomplete the cure.

The adhesive of the invention can be used to bond a variety ofsubstrates together including wood, metal, coated metal, aluminum, avariety of plastic and filled plastic substrates, fiberglass and thelike. In one preferred embodiment, the adhesive is used to bond parts ofautomobiles together or parts to automobiles. Such parts can be steel,coated steel, galvanized steel, aluminum, coated aluminum, plastic andfilled plastic substrates.

An application of particular interest is bonding of automotive framecomponents to each other or to other components. The frame componentsare often metals such as cold rolled steel, galvanized metals, oraluminum. The components that are to be bonded to the frame componentscan also be metals as just described, or can be other metals, plastics,composite materials, and the like.

Adhesion to brittle metals such as galvaneal is of particular interestin the automotive industry. Galvaneal tends to have a zinc-iron surfacethat is somewhat rich in iron content and is brittle for that reason. Aparticular advantage of this invention is that the cured adhesive bondswell to brittle metals such as galvaneal. Another application ofparticular interest is the bonding of aerospace components, particularlyexterior metal components or other metal components that are exposed toambient atmospheric conditions during flight.

Assembled automotive frame members are usually coated with a coatingmaterial that requires a bake cure. The coating is typically baked attemperatures that may range from 140° C. to over 200° C. In such cases,it is often convenient to apply the structural adhesive to the framecomponents, then apply the coating, and cure the adhesive at the sametime the coating is baked and cured.

The adhesive composition once cured preferably has a Young's modulus ofabout 1000 MPa as measured according to DIN EN ISO 527-1. Preferably theYoung's modulus is about 1200 MPa or greater, more preferably at least1500 MPa. Preferably, the cured adhesive demonstrates a tensile strengthof about 20 MPa or greater, more preferably about 25 MPa or greater, andmost preferably about 35 MPa or greater. Preferably, the lap shearstrength of a 1.5 mm thick cured adhesive layer on cold rolled steel(CRS) and galvaneal is about 15 MPa or greater, more preferably about 20MPa or greater, and most preferably about 25 MPa or greater measuredaccording to DIN EN 1465.

The cured adhesive of the invention demonstrates excellent adhesiveproperties (such as lap shear strength and impact peel strength) over arange of temperatures down to −40° C. or lower.

The following examples are provided to illustrate the invention but arenot intended to limit the scope thereof. All parts and percentages areby weight unless otherwise indicated.

TOUGHENER EXAMPLES 1-9 AND COMPARATIVE TOUGHENERS A AND B

A prepolymer is prepared by mixing, under nitrogen, 77.7 parts of a 2000molecular weight polytetrahydrofuran, 0.5 part of trimethylolpropane and0.2 part of a tin catalyst and heating at 85° C. until a homogeneousmixture is obtained. 13 parts hexamethylene diisocyanate are added andthe mixture is allowed to react under nitrogen at 85° C. for 45 minutes.0.01 part of an antioxidant is added, and the mixture is stirred foranother five minutes at 85° C. The resulting prepolymer has anisocyanate content of 3.0%.

The prepolymer is then mixed with 0.1 part of 2-hydroxyethylmethacrylateand 8.7 parts of o-allylphenol under nitrogen. The mixture is allowed tostir for 20 minutes to allow the capping reaction to complete. Theresulting toughener (Example 1) is degassed under vacuum. 1.5% of theterminal isocyanate groups of Toughener Example 1 aremethacrylate-capped, and 98.5% of the terminal isocyanate groups areo-allylphenol-capped.

Toughener Examples 2-9 are made in the same general manner, except thatthe proportions of capping agents are changed in each case, as indicatedin Table 1 below.

Comparative Toughener A is made in the same general manner, except thatall of the terminal isocyanate groups are capped with o-allyl phenol.Comparative Toughener B is made in the same general manner, except thatall of the terminal isocyanate groups are capped with2-hydroxylethylmethacrylate.

Number average and weight average molecular weights for each of thetougheners are determined by gel permeation chromatography. The cappinggroups, molecular weights and polydispersity (M_(W)/M_(n)) for each oftoughener Examples 1-9 and Comparative Tougheners A and B are reportedin Table 1.

TABLE 1 Ex. or Comp. % HEMA¹ % o-allyl phenol Samp. No. capping, mol-%capping, mol-% M_(n) ² M_(w) ² PDI² A* 0 100 5025 11,100 2.21 1 1.5 98.55430 11,610 2.14 2 5 95 5720 11,740 2.05 3 10 90 5375 11,660 2.17 4 2080 5910 12,060 2.05 5 50 50 5715 11,590 2.03 6 80 20 6335 11,790 1.86 790 10 5610 10,930 1.95 8 95 5 5960 11,190 1.88 9 98.5 1.5 5950 11,1701.88 B* 100 0 5835 10,610 1.82 *Not an example of this invention. ¹HEMAis 2-hydroxymethylmethacrylate. ²M_(n) is number average molecularweight of the toughener, M_(w) is weight average molecular weight of thetoughener; PDI is polydispersity, M_(w)/M_(n). Molecular weights are byGPC.

The data in Table 1 indicates that the various tougheners are all verysimilar in molecular weight. There is a trend towards higherpolydispersity with greater o-allyl phenol capping levels.

ADHESIVE EXAMPLES A1-A9 AND COMPARATIVE ADHESIVES C-A AND C-B

One-part, heat activated adhesive formulations are prepared from each ofToughener Examples 1-9 and Comparative Tougheners A and B, using thefollowing formulation:

Component Parts By Weight Diglycidyl ether of bisphenol A 54.5Epoxy-terminated rubber¹ 13.8 Toughener 13.8 Dicyandiamide 4.3Accelerator² 1.3 Fumed Silica 5.4 Fillers/Colorants 5.1 Versatic Acidmonoepoxy ester³ 1.3 Glycidyl silyl ether 0.8 ¹An adduct of acarboxyl-terminated butadiene-acrylonitrile rubber (Hycar ™ X13),bisphenol A based epoxy resin and cashew nut oil. ²Tris(2,4,6-dimethylaminomethyl)phenol in a poly(vinylphenol) matrix.³Cardura ™ E10, available from Christ Chemie.

Adhesive Examples A1-A9 contain Toughener Examples 1-9, respectively.Comparative Adhesive C-A contains Toughener A and Comparative AdhesiveC-B contains Toughener B.

Impact peel testing is performed in accordance with ISO 11343 wedgeimpact method. Testing is performed at an operating speed of 2 m/sec.Impact peel testing is performed at 23° C., and strength in N/mm ismeasured.

Test coupons for the impact peel testing are 90 mm×20 mm with a bondedarea of 30×20 mm. The samples are prepared by wiping them with acetone.A 0.15 mm×10 mm wide Teflon tape is applied to the coupons to define thebond area. The structural adhesive is then applied to the bond area oflatter coupon and squeezed onto the first coupon to prepare each testspecimen. The adhesive layer is 0.2 mm thick. Duplicate samples arecured for 30 minutes at 180° C.

Duplicate test coupons are prepared and are evaluated for lap shearstrength in accordance with DIN EN 1465. Testing is performed at a testspeed of 10 mm/minute. Testing is performed at 23° C. Test samples areprepared using each adhesive. The bonded area in each case is 25×10 mm.The adhesive layer is 0.2 mm thick. Duplicate test specimens are curedat for 30 minutes at 180° C.

Results of lap shear strength an impact peel strength testing on oilycold rolled steel substrates (1.5 mm thick CRS1403 steel coated withRenoform MCO 3028 oil (Fuchs)) are indicated in Table 2. In this and thefollowing examples, failure mode is described as “BF” (“boundaryfailure”), which is a cohesive failure close to the surface of thesubstrate or as “CF” (cohesive failure). Samples which exhibit a mixedfailure mode are described by the percentage of each type of failuremode that is seen.

TABLE 2 testing on oily cold rolled steel substrates Ex. or %- Lap LapImpact Comp. HEMA¹ Shear Shear Peel Sample capping, Str. Failure Str.Impact Peel No. mol-% (MPa) Mode (N/mm) Failure Mode C-A 0 31  CF² 4118% CF/82% BF 1 1.5 31 CF 43 67% CF/33% BF 2 5 28 CF 45 60% CF/40% BF 310 29 CF 36 35% CF/65% BF 4 20 26 CF 31 37% CF/63% BF 5 50 17  BF² 0.4BF 6 80 11 BF 0.4 BF 7 90 11 BF 0.5 BF 8 95 10 BF 0.6 BF 9 98.5 11 BF0.4 BF C-B 100 10 BF 0.3 BF ¹HEMA is 2-hydroxyethylmethacrylate.

The data in Table 2 indicates that on an oily cold rolled steelsubstrate, the presence of some phenol capping in all cases increasesimpact steel strength (relative to Comparative Sample C-B). Lap shearstrength is increased relative to Comparative Sample C-B in all cases inwhich 50% or more of the terminal isocyanate groups are capped with thephenol.

Cohesive failure mode is improved in the lap shear test, relative toboth Comparative Samples C-A and C-B, when from 1.5 to 20% of theterminal isocyanate groups on the toughener are capped with2-hydroxyethylmethacrylate.

In addition, impact peel strength is increased dramatically relative toComparative Sample C-B when from 1.5 to 20% of the terminal groups onthe toughener are capped with 2-hydroxyethylmethacryate. The impact peelstrength is also better than in Comparative Sample C-A, when from 1.5 to5% of the terminal isocyanate groups on the toughener are capped with2-hydroxyethylmethacrylate.

When evaluated on a 0.8-mm oily hot dip galvanized steel substrate (H340LAD+Z, MCO 3028 oil), and on an a 1.3-mm aluminum (AC 120, surfacepretreated with Bonder 299) substrate, Adhesive Examples 1 and 2 performvery similarly to Comparative Adhesive C-A in both lap shear and impactpeel strength testings.

What is claimed is:
 1. A one-part structural adhesive, comprising: A) atleast one epoxy resin including at least one diglycidyl ether of apolyhydric phenol; B) 8 to 30 weight percent of an elastomeric toughenercontaining urethane and/or urea groups, and which has capped terminalisocyanate groups, wherein from 80 to 99% of the isocyanate groups arecapped with a phenol and from 1 to 20% of the isocyanate groups arecapped with at least one hydroxy-functional acrylate or ahydroxy-functional methacrylate compound; C) one or more epoxy curingagents; D) a liquid rubber-modified epoxy resin, a core-shell rubber orboth a liquid rubber-modified epoxy resin and a core-shell rubber; E) acuring agent F) a catalyst, and G) a latent free radical initiator thatis activated by radiation or by heating to a temperature of from 80 to130° C., wherein the adhesive cures rapidly at a temperature of 140° C.or higher but slowly if at all at a temperature of 80° C. or lower. 2.The structural adhesive of claim 1, which further contains at least onemonomeric or oligomeric, addition polymerizable, ethylenicallyunsaturated material having a molecular weight of less than about 1500.3. A method comprising applying the structural adhesive of claim 1 tothe surfaces of two metal members, and curing the structural adhesive toform an adhesive bond between the two metal members, wherein the curingis performed by first heating the structural adhesive to radiation or afirst temperature sufficient to effect a free radical polymerization ofethylenically unsaturated groups and partially cure the adhesive, andthen heating the partially cured structural adhesive to a higher secondtemperature at which the epoxide groups react with the epoxy curingagent.